1. Field of the Invention
The invention relates to a method for manufacturing a solid electrolytic capacitor having conducting polymer as a solid electrolyte, and more particularly to a method for manufacturing such a solid electrolytic capacitor having a superior frequency characteristic.
2. Description of the Related Art
In general, a solid electrolytic capacitor includes a first electrode or an anode composed of a porous compact of valve action metal such as tantalum (Ta) and aluminum (Al), a dielectric substance composed of an oxide film of the anode, and a second electrode or a cathode composed of a solid electrolyte such as manganese dioxide (MnO.sub.2) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) complex salt. The solid electrolyte is required to have functions for electrically connecting an entire dielectric substance disposed in the porous compact to leads of the electrodes, and for repairing electrical short-circuit derived from defects of the dielectric oxide film. Accordingly, it is not allowed to use metal having high electrical conductivity but having no function for repairing an electrical short-circuit as a solid electrolyte. For instance, manganese dioxide (MnO.sub.2), which can be converted to insulative material due to heat generated by short-circuit current, has been often used as a solid electrolyte.
However, a solid electrolytic capacitor having manganese dioxide (MnO.sub.2) as a part of an electrode cannot have a sufficiently large electrical conductivity, and hence has to have a large impedance in the high frequency band. In addition, such a solid electrolytic capacitor can have only a poor thermal resistance, because TCNQ complex salt is easily thermally decomposed.
Recently, new materials have been developed in the field of high polymer. For instance, there has been developed a conducting polymer including conjugate high polymer such as polyacetylene, poly-p-phenylene, polypyrrole and polyaniline to which is added a dopant or an electron donative and/or electron attractive compound.
For instance, Japanese Unexamined Patent Public Disclosure No. 64-24410 has suggested a method for manufacturing a conducting polymer layer by introducing a monomer of a conducting polymer into a surface of an oxide film which plays a role of dielectric substance in liquid phase, and then adding thereto an oxidizer solution including an organic acid. On the conducting polymer layer are formed electrodes each composed of a graphite layer and a silver paste layer.
Japanese Unexamined Patent Public Disclosure No. 2-74019 has suggested a method of manufacturing a composite layer composed of a high polymer layer and a conducting polymer layer, including the steps of covering an oxide film with a high polymer film in which an oxidizer is distributed in advance, and contacting the oxide film with a monomer of the conducting polymer.
In the aforementioned method for manufacturing a solid electrolytic capacitor, suggested by Japanese Unexamined Patent Public Disclosure No. 64-24410, when the liquid monomer of the conducting polymer formed on the oxide film is made to contact with an oxidizer solution, the liquid monomer is diffused in the oxidizer solution. Hence, a required amount of conducting polymer cannot be produced. Thus, even if the method is applied to an existing capacitor which utilizes oxide film having a greatly enlarged surface, the capacitor can have disadvantageously only a low capacity attainment rate which is defined an actual capacitance value relative to a designed value.
On the other hand, in the method suggested by Japanese Unexamined Patent Public Disclosure No. 2-74019, since an oxidizer is in advance diffused in high polymer film, there would not occur a diffusion of the monomer into an oxidizer. However, the composite layer formed on the oxide film is composed of originally non-conducting polymer film and a conducting polymer, and hence the method has a problem that a resultant capacitor can have only quite a poor frequency characteristic.